Atomic Structure and Mass spectrometer

The Table of charges and masses of subatomic particles.

Particle	Abbreviation	relative charge	Relative mass
Proton	p+	1+	1
Neutron	n	0	1
Electron	e-	1-	1/1836

Relative atomic mass Ar

Is the weighted mean mass of an atom of an element relative to 1/12^th of the mass of an atom of carbon-12

It takes into account:

• The percentage abundance of each isotope

• The relative isotopic mass of each isotope

The steps of mass spectrometer

1. A sample is placed in the mass spectrometer

2. The sample is vaporised and then ionised to form positive ions

3. The ions are accelerated. Heavier ions move slowly and are more difficult to deflect than lighter ions, so the ions of each isotope are separated

4. The ions are detected on a mass spectrum as a mass-to-charge ratio m/z. Each ion reaching the detector adds to the signal, so the greater the abundance, larger the signal.

Mass spectrometer graph

The mass spectrum is a graph that shows the isotopes of an atom.

The Y-axis shows the relative abundance of ions, which is given as a percentage

The X-axis is the mass/charge ratio

The example graph shows there are 3 isotopes at 24, 25 and 26

Molecular Ion equation

molecule —— molecule + e^-

Fragmentation

Fragmentation- is when a molecular ion splits in the process of the mass spectrometer

This results in a smaller, positive fragment ion and a neutral species (which is not detected)

Examples:

C₂H₅OH ——— e^- + C₂H₅OH⁺ ionisation

C₂H₅OH⁺ ——— CH₃ + CH₂OH⁺ Fragmentation